1. Field of the Invention
The invention relates to pressure devices such as differential pressure indicators.
Pressure devices which measure differential pressure across a piston frequently use seals to minimize flow of fluid from a higher pressure face of the piston to a lower pressure face of the piston. Such seals apply a force to the piston and any variation in this force causes difficulty in calibrating such devices within required tolerances and will cause the operational parameters of the device to change with time.
2. Review of the Prior Art
Seals currently employed in such devices include piston rings which can be fitted either to the piston or to a bore in which the piston reciprocates, flexible diaphragms and bellows. However, the resistance to movement of the piston caused by these currently employed seals is variable due to inherent manufacturing tolerances, temperature effects, actual pressure levels, lubrication, friction and stiction. Lubrication, friction and stiction effects can vary significantly with time, particularly with repeated temperature fluctuation and especially with elastomeric seal materials.
Where a small leakage is allowable across the piston, the seal can be omitted, being replaced with a close fitting piston within the cylinder perhaps with a labyrinth seal using grooves in the piston and/or the bore. However, even in this proposal, problems can arise in that even a small leakage across the piston can cause problems in the device or fluid system if continuous over a long period of time.